Semiconductors form the basis of modern electronics. Possessing physical properties that can be selectively modified and controlled between conduction and insulation, semiconductors are essential in most modern electrical devices (e.g., computers, cellular phones, photovoltaic cells, etc.). Group IV semiconductors generally refer to those first four elements in the fourth column of the periodic table: carbon, silicon, germanium and tin.
The ability to deposit semiconductor materials using non-traditional semiconductor technologies such as printing may offer a way to simplify the fabrication process and hence reduce the cost of many modern electrical devices such as solar cells. Like pigment in paint, these semiconductor materials are generally formed as microscopic particles, such as nanoparticles, and temporarily suspended in a colloidal dispersion that may be later deposited on a substrate.
Nanoparticles are generally particles with at least one dimension less than 100 nm. In comparison to a bulk material (>100 nm) which tends to have constant physical properties regardless of its size (e.g., melting temperature, boiling temperature, density, conductivity, etc.), nanoparticles may have physical properties that are size dependent, such as a lower sintering temperature.
In general, nanoparticles may be produced by a variety of techniques such as evaporation (S. Ijima, Jap. J. Appl. Phys. 26, 357 (1987)), gas phase pyrolysis (K. A Littau, P. J. Szajowski, A. J. Muller, A. R. Kortan, L. E. Brus, J. Phys. Chem. 97, 1224 (1993)), gas phase photolysis (J. M. Jasinski and F. K. LeGoues, Chem. Mater. 3, 989 (1991)), electrochemical etching (V. Petrova-Koch et al., Appl. Phys. Lett. 61, 943 (1992)), plasma decomposition of silanes and polysilanes (H. Takagi et al, Appl. Phys. Lett. 56, 2379 (1990)), high pressure liquid phase reduction-oxidation reaction (J. R. Heath, Science 258, 1131 (1992)), etc.
In general, a solar cell converts solar energy directly to DC (direct current) electric energy. Configured as a photodiode, a solar cell permits light to penetrate into the vicinity of metal contacts such that a generated charge carrier (electrons or holes—a lack of electrons) may be extracted as current. And like most other diodes, photodiodes are formed by combining p-type and n-type semiconductors to form a junction. After the addition of passivation and anti-reflection coatings, a layer acting as back surface field and metal contacts (fingers and busbar on the emitter, and pads on the back of the absorber) may be added in order to extract generated carriers. Emitter dopant concentration, in particular, must be optimized for both carrier collection and for contact with the metal electrodes.
In general, a low concentration of dopant atoms within an emitter region will tend result in both a low recombination of carriers (and thus higher solar cell efficiencies—the percentage of solar power that is converted to electricity) and poor electrical contact to metal electrodes. In contrast, a high concentration of dopant atoms will tend to have the opposite result. That is good electrical contact to metal electrodes, but high recombination of carriers (and thus reduced solar cell efficiency). Often, in order to reduce manufacturing costs, a single dopant diffusion is generally used to form the emitter, with a doping concentration selected as a compromise between reducing recombination and improving ohmic contact formation. Consequently, a sub-optimal solar cell efficiency is achieved.
One solution is the use of a selective (or dual-doped) emitter. A selective emitter generally uses a first lightly doped region optimized for low recombination, and a second heavily doped region pattern (of the same dopant type) optimized for low resistance ohmic contact formation. Selective emitters are commonly formed with either multiple diffusion steps in conjunction with diffusion blocking layers, or else formed with multiple dopant sources.
However, since the principal variation between such regions is a difference in dopant atomic concentration, there is generally no visible contrast between the highly and lightly doped regions. Consequently, reliably aligning (axial and/or angular) a metal contact pattern onto the formed highly doped region pattern may be problematic.
Likewise, a similar alignment problem may occur in alternative solar cell configurations, such as a back-contact solar cell. Configured with a set of non-visible counter-doped highly doped patterns, a set of corresponding inter-digitated metal contact patterns must also be reliably aligned.
However, proper alignment generally requires the selection of a reference point and a rotation angle that are used to translate and rotate the metal pattern prior to placing it on the solar cell substrate. A first approach involves the creation of artificial reference points or fiducial marks from which both the heavily doped regions and the metal contacts are deposited. However, the use of fiducial marks generally requires an additional processing step (and tool). Furthermore, since both patterns are independently placed relative to the fiducial marks (and not to each other), tolerance errors are additive. That is, first the heavily doped pattern is defined relative to the fiducial marks within a certain tolerance followed by the metal deposition also positioned relative to the fiducial marks with a different tolerance.
An alternative method involves alignment to the substrate edge, which generally requires that the substrate orientation be kept constant (to minimize errors caused by variations in substrate sizes) between subsequent deposition steps. However, for sequential deposition steps, each deposition tool would need to base all calculations on the same exact edge locations, which may be problematic particularly in high throughput manufacturing. Furthermore, since substrate edge geometries tend to be non-ideal and poorly defined, alignment accuracy may also be problematic.
In view of the foregoing, there is a desire to provide methods distinguishing a set of highly doped regions from a set of lightly doped regions on a silicon substrate.